Data storage disk with information encoded in the position error signal fields

ABSTRACT

There is disclosed a disk for use in a disk drive and a method for making the disk. The position error signal (PES) fields for each track on the disk are written with predetermined data, e.g. servo data, encoded thereon. In the preferred embodiment: a) the data such as the track number is mapped into codewords of an error correction code (ECC); b) bit-vectors are used to encode the symbols for further redundancy; and c) the raw data written in the PES fields is constrained to equalize the analog signal characteristic used by the servo system for positioning. Thus, the invention allows the PES fields to be used as in the prior art while also being used as a source of digital data which is robustly recorded therein. In one aspect of the invention, the track ID&#39;s are written in the PES fields in a selected pattern in which the track ID for each track is recorded in two PES field groups. One of the two PES field groups is centered on the selected track while the other PES field group is offset so that it partially overlaps the adjacent track, as well as the selected track. The PES fields will typically be recorded on the disk at the time of manufacture and not changed thereafter.

CROSS REFERENCE TO RELATED APPLICATION

U.S. patent application docket number SA9-97-012, entitled "DISK DRIVEWITH INFORMATION ENCODED IN THE POSITION ERROR SIGNAL FIELDS", withcommon inventors and commonly assigned was filed concurrently with thisapplication. A serial number will be provided when available. Commonlyassigned U.S. patent application bearing Ser. No. 08/813,775 entitled"METHOD AND APPARATUS FOR ENCODING DIGITAL SERVO INFORMATION IN A SERVOBURST" filed Mar. 7, 1997 is also related to the present application andis hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to data storage devices such as a disk drive, andmore particularly to error detection and correction of servo system dataencoded in position error signals on the disk.

DESCRIPTION OF RELATED ART

A disk drive typically includes one or more disks and a head (for eachdisk surface) mounted on an actuator which is positionable over aselected one of the tracks on the rotating disk. A voice coil motor(VCM) is commonly used to position the actuator on which the sliderscontaining the heads are mounted. The slider moves either radially orarcuately toward or away from the spindle center of the rotating diskwhile trying to find or "seek" to a target track. For any selectedtrack, data is recorded upon the track via a "write path" in the system.Alternatively, data is read from the track via a "read path".

The positioning of the slider is typically controlled by a closed loopservo system. The servo is operative both in placing the read and/orwrite transducers (heads) over a target track initially (SEEKING) and inassuring that the appropriate transducer is following the track whilethe data is being read or written (TRACK FOLLOWING). A portion of eachdisk track is formatted to include at least one and typically severalservo fields to allow the servo system to update its head positionseveral times during each revolution. Each servo field is furtherformatted to include servo data including position error signals (PESs).The PES fields are conventionally bursts of constant amplitudesinusoidal signals which are of equal length and are spaced at fixedintervals. These fields are used to generate analog signals which varywith the position of the read head and allow the precise positioning ofthe heads over the track. The PES typically consists of three or fourstaggered bursts of transitions, for example, an A, B, C and D burst. Ifthe measured amplitude of the bursts indicates that the head is not inposition, then a control signal is generated to the VCM to adjust theposition. A servo system typically includes a servo processor whichcontrols the VCM and monitors the servo data being read from the disk.

One way of organizing disk storage tracks is to divide a track into datasectors, a data sector being a fixed number of bits recorded along thetrack. The servo data and PES information are typically located alongradially arcing lines, but in the commonly used "zone bit recording"(ZBR) method the number and position of the data sectors variesaccording to the distance that the track is from the center of the disk.(See for example, Hetzler, et al. U.S. Pat. No. 5,210,660). Usually,there are several servo sectors per track. This results in at least someof the servo sectors being located inside data sectors forming aso-called "split data field." The servo sectors include the PES andservo data including track identification (TID) and other controlinformation.

Conventionally each track has its unique track identity (TID), i.e. thetrack address, magnetically recorded in the servo data. During a SEEK,while moving radially inward or outward, the servo system reads the TIDsfrom the servo sectors on a track. The TIDs tell the servo system wherethe head is at that moment and allows the system adjust its velocity,etc., to navigate to the target track.

A typical prior art format used on servo sectors comprises the followingfields:

    --||Rd/Wr|AGC"SID|TID|Hd"Cyl.vertline.PES||--.

In this format, the SID (Servo ID) character consists of a timing lineor mark and locates the beginning of servo sector position informationon the track. The TID (track identity) character is typically expressedas a codeword in a reflected binary (also called a "Gray" code). Thevirtue of a Gray code is that the TID number varies from track to trackby only one digit. This assists robust reading of TIDs even whenofftrack as occurs during rapid seeking. HD (head) and CYL (cylinder)fields may provide other information.

In the prior art, there have been suggestions for enriching thePES-stored information. Cunningham, "Quad Burst Servo Needing No Sync IDand Having Added Information", IBM Technical Disclosure Bulletin, Vol.33, No. 3B, pp. 198-200, August 1990, describes a quad burst servopattern in which approximately 8 bits of information can be encoded inthe PES burst by phase manipulation of the fundamental sinusoidalfrequency which is used by the servo system. However, the phase methodresults in a loss of amplitude for normal servo detection. Theinformation can include data unique to each burst identifying it as A,B, C or D, for example by using two bits, as well as some of the lowerorder bits of the track or sector number.

Similarly, Andresen, U.S. Pat. No. 4,195,320, "Record TrackIdentification and Following", issued Mar. 25, 1980, discloses a servosystem for a floppy disk which uses track address information digitallyencoded in servo bursts using a sliding modulo code and also integratesthe signals to obtain an analog measure of the read transducer'sposition in relation to the track.

While enrichment of PES fields and the like in the servo sector canresult in enhanced device access performance, it renders the accuratepositioning of the head/transducer over the target track vulnerablewhere the servo sector information is susceptible to corruption by erroror erasure. The corruption may occur randomly or intermittently. Thesources may be induced noise, phase jitter, or the like.

SUMMARY OF THE INVENTION

A data storage system with an improved servo system and an improveddisk, as well as, a method of recording and retrieving servo informationwith protection from errors is described herein. The method of theinvention involves mapping the predetermined data, e.g. servo data, intocodewords with redundancy information such as in an error correctioncode (ECC) and then recording these codewords into the PES fields. ThePES fields will typically be recorded on the disk at the time ofmanufacture and will not be changed thereafter. The preferred ECCencoding method is selected to maintain analog signal characteristicsregardless of the data stored in the PES, so that minimal, if any,change in the conventional servo detection circuitry is needed use thePES bursts as in the prior art, as well as, sources of digital data.Thus, the codewords of the ECC are constrained in the allowed weights,e.g., the number of 1's in each codeword (or magnetic transitions), andrun-length-limited (RLL) attributes.

In a preferred embodiment the servo data mapped into the ECC codewordsincludes track identification (TID) which can be represented by a fixednumber of bits of information. The bits are converted into one or moreECC codewords. Additional constraints such as equal weight are placed onthe code by mapping the data comprising the TID into limited set of bitpatterns (bit-vectors). If disallowed bit patterns are read back, thenan error is detected independently from the ECC syndrome calculationprocess. Placing the TID information in the PES fields using the robustmethod of the invention optionally allows the conventional TID field inthe servo data to be reduced in size or eliminated with a resultingincrease in recording capacity. Although various types of ECC algorithmscan be used, either a Reed-Solomon type ECC or a Hamming type ECC arepreferred.

In one aspect of the invention, the track ID's are written in the PESfields in a selected pattern in which the track ID for each track isrecorded in more than one PES field in more than one group of PESfields. One group of PES fields is centered on the selected track whilethe other group of PES fields is offset so that it partially overlapsthe adjacent track, as well as the selected track. When the drive isoperating, the previously recorded ECC codewords in one or more of thePES signals is read and the constrained symbols and the ECC codewordsare used to ascertain the presence of errors. If errors are found,correction can be performed up to the limit of the selected code. Thecorrections are made by computing the syndromes as in ECC prior art.This gives an estimate of the servo data that includes the TID. The TIDused in conjunction with the waveforms induced in the read head by thePES gives a precise measure of the position of the head on the disk.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts a block diagram of a disk drive expressing the major dataflow and control flow therein according to the prior art.

FIG. 2A shows a typical prior art servo sector format with fielddesignations and bit lengths.

FIG. 2B illustrates a servo sector reformatted with field designationsand bit lengths according to the invention.

FIG. 3 depicts an example according to the invention of encodingcombined TID and PES values utilizing a constrained Reed-Solomon errorcorrection code according to the invention.

FIG. 4 illustrates the logic processing blocks in a PES data processoraccording to an embodiment of the invention.

FIG. 5 illustrates the positions of the PES bursts in relation to thedata track centerlines and track ID content of each burst on a disk inan embodiment of the invention.

FIG. 6 is a block diagram of a PES data processor according to anembodiment of the invention.

FIG. 7 is a block diagram showing a PES data detector and PES dataprocessor in relation to other servo system components.

FIG. 8 illustrates the error registers and track ID registers in asecond embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment described below is merely illustrative and is notintended to be limiting, the latter being the function of the claimsappending the specification.

Referring now to FIG. 1, there is shown a simplified block diagram of adisk drive illustrating the major components, data flow and control flowtherein according to the prior art. A modem disk drive may have one ormore microprocessors controlling various functions. FIG. 1 shows adesign with a single microprocessor 116. The program for themicroprocessor is commonly called the firmware. Commands and data forthe drive come in from a host device (not shown) via path 162 into theinterface electronics 114, where the host level commands are translatedinto device level commands and actions. The controller electronics 115coordinates activities and data flow to and from the Read/Write (R/W)Electronics 113. The Servo Electronics 112 (which typically includes aservo processor) receives raw position information from the read head(s)108, including the signal induced by the PES bursts and the signalcorresponding to the TID, then controls the movement of the actuator 106through electrical signals sent to the voice coil motor (VCM) 110. Theheads 108 are mounted on the end of the actuator 106. The disk 104 isrotated by a spindle motor (not shown) around spindle 111. The slightlyarcing servo sectors 120A, 120B contain the servo data and PES bursts.The data is written in circular tracks 118 on the planar surface disk.The tracks are divided into data sectors 154A, 154B where the user orcustomer data is stored.

The R/W Electronics perform the basic demodulation and detectiondecisions on the raw data signals from the read heads and convert thesesignals into binary data (a stream of 1's and 0's). The raw data signalswritten onto the disk are constrained according to schemes such asrun-length limited (RLL) coding which enhance the ability of the systemto accurately read the data back. The converted binary data are storedin a buffer (not shown). At this stage the data in the buffer includesredundancy information typically called ECC check bytes or symbols. TheECC check bytes together with the true data (typically called host orcustomer data) bytes form ECC codewords. The ECC detection andcorrection hardware and or firmware is commonly considered to be acontroller function. When no errors are found or when the errors havebeen corrected, the customer data is sent to the host without the ECCbytes. It should be noted that the servo related information, i.e., theTID and burst signals are not typically sent to the host.

The disk 104 has a thin film magnetic coating which allows the servoinformation and the raw data to be stored as magnetic transitions. Theservo sector data 120A-B are typically written in arcing radial lines atregular radial (angular) positions around the disk. For purposes of thisinvention, a sector is defined as a fixed recording length along atrack. For instance, a typical data sector might be 512 bytes in length.The data sectors 154A, 154B are placed somewhat independently of theservo sectors, so that there may be multiple data sectors between twoservo sectors and servo sectors may fall in the middle of a data sectorforming what are called split data fields, e.g. 154B. If the tracks of adisk are formatted under ZBR as mentioned in the Hetzler '660 patent,then tracks in outer zones 151, 152 would have more data sectors betweenservo sectors than the tracks in inner zones 153. Although only threezones are shown in FIG. 1, there is no arbitrary limit on the number ofzones.

FIG. 7 is a block diagram showing a PES data detector and PES dataprocessor in relation to analog servo processing circuitry and a servoprocessor that illustrates the signal and data flow at a high level in adisk drive embodying the invention. The waveforms from the read headwhich are induced by the PES bursts are processed in two parallel paths.The waveforms can be processed as in the prior art by analog servoprocessing circuitry 35 which uses the relative analog signal strengthsinduced by the bursts to provides information to the servo processor 34on the position of the head in relation to a track. In addition to theanalog processing, the waveforms are converted by the PES data detector31 into raw binary data. The binary data is then processed to recoverthe track ID (and any other information encoded into the PES fields) bythe PES data processor 33. The track ID information, etc., is thenprovided to the servo processor 34 for use in controlling and adjustingthe position of the heads, e.g. seeking and track following.

The invention is independent of the servo data architecture, but forbetter understanding of the invention and not to imply limitation, aprior art servo sector format will be compared to an embodiment of theinvention. There is shown in FIG. 2A a typical prior art servo sectorformat with field designations and equivalent bit lengths. The totallength of the servo sector is 134 bits and it is partitioned into sevenfields. The effective length is only 124 bits since the W/R field is notpertinent to positioning information. The SID (Servo ID) field consistsof a unique bit pattern and marks the start of the servo sector proper.Of the next three fields; namely, HD (Head), TID (Track Identity), andSECTOR, the TID is the most critical since it identifies the track. TheSECTOR and HD provide ancillary positioning data. The practice has beento encode just the TID field in a reflected binary code. The virtue ofreflected binary or Gray code is that the track identity from track totrack changes by only one bit. This permits off-track reading of the TIDfield. The property of changing just one bit between tracks when usingthe Gray code has, prior to the instant invention, precluded protectingthe TID and other information in the servo field with ECC.

FIG. 5 illustrates the relative positions of the PES fields, which areoften called "bursts," with regard to each other and the centerlines ofthe tracks in an embodiment of the invention. The terms "field, "signal"and "burst" will be used interchangeably to refer to a PES unit. Tracks1-5 are shown as extending horizontally across the figure with theircorresponding PES bursts. The head will be assumed to be traveling fromleft to right as FIG. 5 is drawn The PES bursts are shown as beingapproximately as wide as the tracks, but the precise width andpositioning are not critical for practicing the invention. Some of thebursts are placed on the centerline of the track. These will be referredto a one group of bursts while the bursts which are offset from thecenterlines will be referred to a second group. The PES burst 51 iscentered on the centerline of track 1. PES burst 52 is offset from thecenterline of track 1, i.e. above the centerline, so that less than onlya portion of the width of burst 52 is over track 1. PES burst 53 isoffset below the centerline of track 1, so that only a portion of thewidth of burst 53 is over track 1. PES burst 54 is centered on track 2.PES bursts 51-54 are also placed at different positions along the trackso that the head following track will pass each of the burst at adifferent time. PES burst 53 will be passed first, PES burst 52 second,PES burst 54 third and PES burst 51 last. Thus, the PES bursts in FIG. 5are arranged in four distinct time positions when the disk is rotatingunder the head. Interchangeably the four PES groups will be described asfalling into four radial positions. The groups of four PES bursts willbe repeated multiple times around the disk as shown in FIG. 1 and willbe repeated for each pair of concentric tracks as in the prior art. Theorder of the bursts is not critical for the invention. For example, thecenterline bursts 51, 54, etc. (Group A) could be placed ahead of theoffset bursts 52, 53, 55, etc. (Group B). The system can use therelative signal strength of the waveform induced in the read head fromeach of the four bursts to judge where the head is in relation to thecenterline as per the prior art, since as will be described below, theanalog characteristics of bursts are kept essentially constant despitehaving information encoded in them.

The inventions described herein allow information, including track IDinformation, to be stored in the PES fields. Continuing to referenceFIG. 5, in the embodiment of the invention shown, the track ID for track1 is recorded in both PES bursts 52 and 51. Similarly, the track ID fortrack 2 is recorded in both PES bursts 53 and 54. When a head is eitherseeking or track following, the relative signal strength at the headfrom the bursts will vary. It is to be expected that it will be moredifficult to read the information recorded in bursts as the signalstrength at the head decreases. Since the burst are arrayed in anoverlapping pattern, the head will be able to read at least one andperhaps more of the track ID fields.

In the following example it will be assumed that a head following atrack will typically be able to read the information in the threenearest bursts, but this is not required for the invention to besuccessfully employed as will be seen. A head following track 1 in FIG.5 will pass burst 53 and read track ID 2, then pass burst 52 and readtrack ID 1, and will then pass over burst 51 and read track ID 1. Ifthese IDs are concatenated, they are "211." A head following track 2will pass burst 53 and read track ID 2, then pass burst 55 and readtrack ID 3, and will then pass over burst 54 and read track ID 2. Ifthese IDs are concatenated, they are "232." This pattern will berepeated for each pair of tracks making an even/odd track distinction.Thus, the track 1 ID sequence is "n+1, n, n" while the track 2 sequenceis "n, n+1, n." In either case the servo system can determine whichtrack it is following despite having read two different track ID's.

If the system is unable to read the track ID in one or more of thebursts, position information can still be gleaned by knowing the timingof the burst(s) and the ID(s) encoded therein. For example, if the headis able to read only the track ID from burst 51 in the last time slot,the servo system will know that it has read the centerline burst fortrack 1. If only burst 52 has been read, the drive will know the trackID is 1 and that the burst to the left of track 1 was read. If onlyburst 53 has been read, the servo system will know the track ID is 2 andknow that the burst overlaying track 1 and track 2 was read. Thus,combining the track ID with knowledge of which timing slot the burst wasread gives not only the track ID itself, but additional positioninformation as well.

The technique for recording the track ID as well as any otherinformation in the PES burst will now be illustrated by describing aspecific embodiment of the invention. In FIG. 2A, the PES (PositionError Signal) field of 64 bits is the largest. The 64 bit field shouldbe thought of as space reserved for the PES bursts, since only asubportion, i.e, one burst will be aligned with the track and the restwill either be absent or offset from the track centerline as isillustrated in FIG. 5. The prior art bursts, of which there are two,three or four, are essentially a group of uniformly spaced magnetictransitions which can be thought of as a sequence of recorded 1's. Thetwo, three or four bursts are identical except for their position. It isnoted that this prior art servo sector format would typically be writtenphase-aligned track to track at a nominal 20 MHz rate.

FIG. 2B shows an embodiment of a servo sector formatted with fielddesignations and bit lengths according to the invention. It should benoted that the servo sector is larger, consisting of 212 bits in length.However, the improved error tolerance permits recording information inhigher bit densities so less total disk space is used. Also, there areseveral substantive changes. The ECC for the PES described below is notto be confused with the standard ECC which is used to protect thecustomer data. The customer data ECC is not effected in anyway in adrive using the PES ECC of the invention.

At the initial, lowest level magnetic transitions in the PES signals aretranslated into binary data bits. In FIG. 7 the unit which takes thewaveform from the read head and performs this function is shown as thePES data detector 31. The bits of information can be encoded in the PESin several ways and the chosen method has no necessary relationship tothe scheme used for the customer data. The following are examples ofprior art ways to record the bits, but are not intended to be anexhaustive list. A bit can be mapped as one "data" bit to one fluxtransition, in which case the presence of a flux transition at aparticular time is a one and its absence a zero. This provides the mostcompact scheme of encoding. For example, let "+" be a positive fluxtransition, "-" a negative transition, and "0" no transition, then"1101" becomes "+-0+". A bit can be mapped as one "data" bit to a pairof flux transitions (dibit); again the presence of a dibit is a one andits absence is a zero. This is less compact, but dibits are often easierto detect than single bits because of channel filtering. In this case,"1101" becomes "+-+-00+-", twice as long as the first example. The"data" bit can be encoded using three clocking periods, the first ofwhich has a flux transition (for clocking/run length limiting) and thesecond or third contains a second flux transition depending on whetherthe "data" is a one or zero. In this case "1101" becomes "+-0+-0+0-+-0".This particular scheme is not especially advantageous for use with acode that provides run length limiting.

It is also possible to use the scheme proposed by Cunningham in thepreviously cited publication. If the data is phase encoded, using thesame notation as above a 1 becomes "0+00" or "0-00" and a "0" becomes"00+0" or "00-0". This is useful when the bit frequency is substantiallylower than the demodulator's clock.

Given a scheme to record and decode binary data in the PES, the actualinformation content and ECC of the PES can be designed. In the describedembodiment the servo data is mapped into an ECC codeword for any giventrack by assigning an m*n bit string to the TID and partitioning the m*nbit string into m bytes of length n bits each. Next, r redundant bytesof n bits each are formed as the residue of an ECC encoding process overthe m*n bit string. After determining the r redundant bytes, each n bitbyte for any given (m*n+r*n) bit string is mapped into an (n+q) bitvector of weight j. Codewords in the ECC code are formed byconcatenating the 2^(n) vectors in a predetermined order. Finally, eachcodeword is written as signal bursts of 1's and 0's into the PESpositions of the formatted servo field.

For example, if the parameters m, n, r, q, and j assume the values ofm=6 bytes, n=3 bits per byte, r=2 redundant bytes, (n+q)=5 bits, j=3,then m(n+q)+r(n+q)=40 bits per codeword, and codeword weight=(m+r)j=24.if the TID is a string of m*n=18 bits, firstly, the string will beblocked into m=6 bytes of n=3 bits per byte. Secondly, r=2 redundantbytes of n=3 bits each are derived as a residue of a Reed-Solomon orHamming ECC encoding operation over the m*n=18 bit string. Thirdly, eachn=3 bit byte for any given string of (m en+r*n)=24 bits=8 bytes ismapped into an (n+q)=5 bit vector of weightj=3. The mapping excludesvectors having runs of j leading or trailing 1's, such as 11100 and00111. A codeword in an ECC code as used in this invention is aconcatenation of the 2^(n) =8 vectors of 5 bits each. This constitutes a40-bit codeword in the ECC code. This codeword is written into one ormore of the PES fields. Each codeword thus exhibits a predeterminedminimum weight and run-length-limited attributes.

In this embodiment of the invention for a 40 bit codeword, the code ofchoice is a [40,18,6] constant weight 24 code. Other codes are equallypossible depending of the length of the TID field, the degree ofprotection desired, etc. The example code has a length 40, a dimension18, a minimum distance 6, a constant weight 24, and a maximum run of 0'sof 2. The dimension 18 is the bit length of the data to be encoded. Thelength 40 is the number of bits in each codeword. The weight 24 is thenumber of 1's in each codeword. The constant weight of the code allowsthe burst signals detected by the head to be integrated and used as inthe prior art, since each burst will have 24 bits which are 1's and 16bits which are 0's in varying positions.

As mentioned previously, the track identity TID in this embodiment isrepresented by a string of 18 bits. This TID string is partitioned intosix 3-bit symbols. Each symbol is an element in a Galois field GF (8) asgenerated by a primitive polynomial, e.g. (1+x² +x³). To the sixinformation symbols, two redundant symbols are added using the paritycheck matrix: ##EQU1## The finite field GF(8) defined by 1+x² +x³ can bewritten according to Table 1 as follows:

                  TABLE 1                                                         ______________________________________                                        Value of "α"                                                                          Power of "α"                                              ______________________________________                                        000           0.sup.                                                            100 1.sup.                                                                    010 α.sup.                                                              001 α.sup.2                                                             101 α.sup.3                                                             111 α.sup.4                                                             110 α.sup.5                                                             011 α.sup.6                                                           ______________________________________                                    

Next, each 3-bit symbol is transformed into a 5-bit symbol (bit-vector)or codeword of weight 3 according to Table 2 as follows:

                  TABLE 2                                                         ______________________________________                                                      5-bit symbol                                                      3-bit symbol X Vector Y = f(X)                                              ______________________________________                                        000           10110                                                             001 11010                                                                     010 11001                                                                     011 10101                                                                     100 10011                                                                     101 01110                                                                     110 01101                                                                     111 01011                                                                   ______________________________________                                    

Assume that it is desired to encode the 18-bit string u in the form ofsix 3-bit bytes:

    Let u=011 000 100 010 010 110.

If the bytes in u are written as powers of "a" according to Table 1,then

    u=(a.sup.6, 0,1, a,a,a.sup.5).

Encoding u using the parity check matrix H, then two redundant bytes areadded to u as follows:

    v=(a.sup.6, 0,1, a,a,a.sup.5,a.sup.2,a.sup.5)=011 000 100 010 010 110 001 110.

Finally, each of the eight 3-bit symbols of "v" is mapped into a 5-bitvector of weight 3 according to Table 2 to obtain the final constrainedRS codeword "w":

    w=10101 10110 10011 11001 11001 01101 11010 01101.

Aspects of Decoding According to the Invention

The embodiment example used for encoding will be used to illustrate datadecoding according to the invention. After the binary codeword has beenread back from the PES field(s) the first step partitions the codewordinto the 5-bit vectors Y which are then mapped into a 3-bit symbols Xaccording to Table 2. Thus, f⁻¹ (Y)=X denotes this mapping. However, itmay happen that due to errors in a particular 5-bit vector, there is nocounterpart 3-bit symbol. In this case, if Y is not in the table, thenthe value "000" is assigned to f⁻¹ (Y) and at the same time the symbolis flagged as being in error, i.e. a corresponding error flag is set.

Assume that a word W of length 40 is read back from the PES field of theservo sector from a particular track. In the above notation, these wouldbe (Y₀ Y₁ Y₂ Y₃ Y₄ Y₅ Y₆ Y₇), where each Y_(i) is a 5-bit vector. Thenext step in decoding is looking up each vector Y in the table andcounting the number of error flags . This is termed estimating the valueR.sup.(i) of each vector Y_(i) and is denoted by R.sup.(i) =f⁻¹ (Y_(i))lying in the range (0≦i≦7).

Since the codewords in this embodiment have a minimum distance of sixfrom each other, this means that the code can correct up to two errorsand detect up to three errors. If three or more flags have been countedor set, then at least three errors have occurred and the vector isuncorrectable. However, if less than three flags have been counted, thenthe error, if any, may be correctable. If zero flags are counted thereis still a possibility that errors exist. If there are two or lessflags, the next step is to derive the syndromes S from the eight symbolsR.sup.(i) utilizing the parity check matrix H:

    S.sub.0 =R.sup.(0) ⊕R.sup.(1) ⊕R.sup.(2) ⊕R.sup.(3) ⊕R.sup.(4) ⊕R.sup.(5) ⊕R.sup.(6) S.sub.1 =R.sup.(0) ⊕aR.sup.(1) ⊕a.sup.2 R.sup.(2) ⊕a.sup.3 R.sup.(3) ⊕a.sup.4 R.sup.(4) ⊕a.sup.5 R.sup.(5) ⊕R.sup.(7)

The remaining part of the analysis is standard syndrome interpretationof RS codewords in three modes; namely, no flags, one flag, or twoflags.

Referring now to FIG. 3, there is shown a visual example according tothe invention of encoding a TID utilizing a constrained RS errorcorrection code and writing the codeword into PES fields. Thus, thecodeword is utilized for both track identification and position errorsignal purposes, and in that sense serves a combined function. In thisexample, the TID for track 10 is represented by an 18-bit sequence. The18-bit sequence is in turn parsed to form six 3-bit symbols. Tworedundant 3-bit symbols are added using an extended [8,6] Reed-Solomoncode as defined by parity check matrix H. The six symbols plus the tworedundant symbols are each in turn mapped into counterpart 5-bit vectorsof the [8,6] constrained Reed-Solomon code. As mentioned before,concatenation of the eight vectors in a predetermined order forms acodeword suitable for recording in the PES fields.

Referring now to FIG. 4, there is shown an embodiment of a PES dataprocessor. The apparatus shown in FIG. 4 is conceptually part of theservo electronics (FIG. 1, 112). It includes four pattern registers 156,158, 160, 162 and an arrangement of timing 154 and selector circuits 150responsive to the standard SID 102, SELECT 106 and CLOCK 104 signals.The PES data detector 31 converts the magnetic transitions in the PESfields into data bits which flow on data path 100 into a shift register152. The shift register synchronously transfers RS-coded patterns onpath 114 received on the data path 100 using clock 104 and select line110 to the selected one of a plurality of pattern registers 156,158,160, 162. The selector line 110 from the selector 150 to the patternregisters comprises at least two bits to allow selection of any one ofthe four pattern registers. A symbol processor 164, inputs the selectedoutput from one of the pattern registers over a connecting path 118,then performs the bit-vector to symbol mapping and the RS detection andcorrection on codewords in the selected pattern. Further, the symbolprocessor 164 corrects any errors in the patterns up to the capacity ofthe code and writes the corrected TID on path 120 into a register 166and the error flags 124 (or number of errors) to register 168 on path124 at a time determined by the output of the control register 154 usingsignal 116. The TID register 166 and error register 168 are thenavailable for use by the servo system over paths 122 and 126respectively.

The track ID decode process begins with the servo ID (SID) timing signalon path 102 initializing timing control circuitry 154. A gate signal onpath 112 from the timing circuit 154 controls the flow of RS codewordson path 100 into a shift register as paced by clocking on path 104. TheRS codewords are then stored in the pattern registers 156, 158, 160, 162as clocked by timing signals on path 110 from timing circuit 154. Sincethe four different groups of PES bursts are distinguishable by theirradial positions on the disk in relation to the SID, they are alsodistinguishable by the time that they pass by the read head as the diskrotates.

Following the capture and storage of the RS codewords in the registers,the servo electronics will select one of the four RS codeword patternscorresponding to one of the four PES burst groups. A 2-bit PES patternselection signal on path 106 is supplied to circuit 150. In turn,circuit 150 activates one of the four select lines 108 to enable outputof one of the pattern registers 156, 158, 160, 162. The pattern registerRS codeword contents are applied to symbol processor 164 over path 118where its syndromes will be derived and any errors determined from thesyndromes. Errors within the power of the code will be corrected andreported in error register 168. This will be accompanied by anindication of the number of errors. Otherwise, the TID will be stored inregister 166.

FIG. 6 is a block diagram of a symbol processor 164 according to thedescribed embodiment of the invention. The raw data from a selected oneof the PES pattern registers comes into the bit-vector extractor 61which partitions the bits into the bit-vectors, e.g. 40 raw bits into 8vectors of 5 bits each. The bit-vectors are passed to the mapper 62which maps the 5-bit vectors using the vector:symbol table 63 into thecorresponding 3-bit symbols for all 5-bit vectors not containing errors(symbol "000" is used for the vector having errors). One of the errorflags 64 is set or a counter is incremented for each 5-bit vector notfound in the vector:symbol table 63. The 3-bit symbols are passed to thesyndrome calculator 65. The syndromes are passed to thedecoder/corrector 67 which uses the syndromes to detect and correcterrors up to the code capability and place the corrected symbols in thetrack ID register 166. The decoder/corrector 67 has access to the flags64. If there are no flags (i.e., error count is equal to zero) and thesyndromes are zero, then the 3-bit symbols are placed unchanged into thetrack ID register 166. The final error status is updated in errorregister 168 as previously indicated.

A second embodiment of the invention uses four error registers (A-D) andfour track ID registers (A-D), as illustrated in FIG. 8, which replacethe single error register and single track ID register in the embodimentshown in FIG. 4. Given sufficiently fast circuitry and/or processingspeed, the symbol processor could attempt to resolve the track ID andother information in all four PES burst groups and store each result ina corresponding track ID register. The output 116 of the timing controlcircuit 154 of FIG. 4 could be used in conjunction with selector circuitoutput 108 (not shown in FIG. 8) to process and store the informationfrom each of the four PES bursts (A,B,C,D) in the corresponding track IDregister (A-D). Similarly in this embodiment there are fourcorresponding error registers (A-D) which are selected to store theerror status for each of the four bursts. Note that it is not necessarythat all four PES bursts be successfully read for a particular position.Preferably each of the track ID registers and the error registers wouldbe made available for reading by a servo processor. Given the method ofrecording the track ID's in the PES fields shown in FIG. 5, multipletrack ID registers would allow easy comparison of the ID's from eachfield so that the maximum information about the position would be passedto the servo processor.

While the invention has been described with respect to an illustrativeembodiment thereof, it will be understood that various changes may bemade in the method and means herein described without departing from thescope and teaching of the invention. Accordingly, the describedembodiment is to be considered merely exemplary and the invention is notto be limited except as specified in the attached claims.

What is claimed is:
 1. A data storage disk comprising:a substrate; amagnetic recording medium deposited on the substrate; and a plurality ofposition error signals recorded on the magnetic recording medium, theposition error signals having at least first and second groups with thefirst group of position error signals being angularly offset from thesecond group of position error signals, the first group being positionedon a centerline of a track and the second group being offset from thecenterline of the track, each position error signal (PES) being a set ofmagnetic transitions, each set of magnetic transitions representingbinary data which is a function of a track number where the PES islocated, the binary data including redundancy bits which provide a firsterror detecting scheme for reading errors, the binary data being furthercomposed of a plurality of bit-vectors which correspond to symbolshaving fewer bits than the bit-vectors which creates a mapping providinga second error detecting scheme.
 2. The data storage disk of claim 1wherein the binary data in first and second groups of PES includes thetrack number on which the first group of PES is centered.
 3. The datastorage disk of claim 2 further comprising third and fourth groups ofposition error signals which are angularly offset from each other andfrom the first and second groups of position error signals, the firstgroup of PES being located on the centerline of alternating tracks whilethe third group of PES is located on the centerline of tracks locatedbetween the alternating tracks and the fourth group of PES is offsetfrom the centerline of the third group of PES.
 4. The data storage diskof claim 1 wherein the binary data in first and second groups of PESincludes ECC check symbols.
 5. The data storage disk of claim 1 whereinat least one symbol represents an ECC check symbol.
 6. The data storagedisk of claim 1 wherein the binary data in first and second groups ofPES is composed of a plurality of n-bit vectors which map to a pluralityof symbols having n-2 or less bits per symbol.
 7. The data storage diskof claim 6 wherein the plurality of symbols form at least one codewordin an error correction code.
 8. The data storage disk of claim 1 whereineach PES in the first group has an equal number of magnetic transitions.9. The data storage disk of claim 8 wherein the binary data in first andsecond groups of PES includes the track number on which the first groupof PES is centered.
 10. The data storage disk of claim 9 furthercomprising third and fourth groups of position error signals which areangularly offset from each other and from the first and second groups ofposition error signals, the first group of PES being located on thecenterline of alternating tracks while the third group of PES is locatedon the centerline of tracks located between the alternating tracks andthe fourth group of PES is offset from the centerline of the third groupof PES.
 11. A data storage disk comprising:a substrate; a magneticrecording medium deposited on the substrate; and a plurality of positionerror signals recorded on the magnetic recording medium, the positionerror signals having at least first, second, third and fourth groupswith each group of position error signal (PES) having a uniquelyidentifying angular offset from a servo index marker, the first andthird groups of PES being respectively positioned on centerlines ofalternate tracks and the second and fourth groups of PES being offsetfrom centerlines of the tracks, each PES being a set of magnetictransitions, each set of magnetic transitions representing binary datawhich is a function of a concentric track number where the PES islocated, the binary data including redundancy bits for detecting readingerrors, the binary data being further composed of a plurality of n-bitvectors which map to a plurality of symbols having n-1 or less bits persymbol, the binary data for the first and second PES groups containinginformation which identifies the track on which the first PES group iscentered and the binary data for the third and fourth PES groupscontaining information which identifies the track on which the third PESgroup is centered.
 12. The data storage disk of claim 11 wherein theplurality of symbols form at least one codeword in an error correctioncode.
 13. The data storage disk of claim 11 wherein the binary dataforms at least one codeword in an error correction code.
 14. A method ofmaking a data storage disk having a magnetic recording medium comprisingthe steps of:encoding a track identifier n in a first codeword whichincludes redundancy bits; dividing the first codeword into a first setof one or more symbols; mapping the first set of one or more symbolsinto a set of bit-vectors having more bits per vector than the number ofbits in a symbol; and writing the the bit-vectors in a plurality ofposition error signals at selected radial positions on the magneticrecording medium.
 15. The method of claim 14, wherein the bit vectorshave equal weight.
 16. The method of claim 14, the encoding stepsfurther comprising the steps of:calculating error correction codecheckbytes; and including the checkbytes in the codeword.